Question #283958
  1. in R3, let U span (1,0,0),(0,1/root2, 1/root2). then U is an element of U such that ||u-(2,4,6)|| is as small as possible.
  2. for a given function f:R to R defined as f(x)=2x-1, the image of S={x is an element of R: x^2-4>/=0} is?
  3. suppose T: R^2 to M22 is a linear defined by T(U,V)=[(U,U), (V,2U)]. Then ker(T) is?
  4. suppose T:R^6 to R^4 is a linear map such that null T=U, where U is 2-dimensional subspace of R^6. Then dim range T is?
1
Expert's answer
2022-01-04T10:32:24-0500

1.

A=(1001/201/2),b=(246)A=\begin{pmatrix} 1 & 0 \\ 0 & 1/\sqrt 2\\ 0&1/\sqrt 2 \end{pmatrix},b=\begin{pmatrix} 2 \\ 4\\ 6 \end{pmatrix}

Then u is the orthogonal projection of b onto the subspace spanned by the column of A and it is given by the formula:

u=A(ATA)1ATbu=A(A^TA)^{-1}A^Tb


then:

ATb=(10001/21/2)(246)=(210/2)A^Tb=\begin{pmatrix} 1 & 0&0 \\ 0 & 1/\sqrt 2& 1/\sqrt 2\\ \end{pmatrix}\begin{pmatrix} 2 \\ 4\\ 6 \end{pmatrix}=\begin{pmatrix} 2 \\ 10/\sqrt 2 \end{pmatrix}


ATA=(10001/21/2)(1001/201/2)=(1001)A^TA=\begin{pmatrix} 1 & 0&0 \\ 0 & 1/\sqrt 2& 1/\sqrt 2\\ \end{pmatrix}\begin{pmatrix} 1 & 0 \\ 0 & 1/\sqrt 2\\ 0&1/\sqrt 2 \end{pmatrix}=\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}


(ATA)1=(1001)(A^TA)^{-1}=\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}


(ATA)1ATb=(1001)(210/2)=(210/2)(A^TA)^{-1}A^Tb=\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}\begin{pmatrix} 2 \\ 10/\sqrt 2 \end{pmatrix}=\begin{pmatrix} 2 \\ 10/\sqrt 2 \end{pmatrix}


u=(1001/201/2)(210/2)=(255)u=\begin{pmatrix} 1 & 0 \\ 0 & 1/\sqrt 2\\ 0&1/\sqrt 2 \end{pmatrix}\begin{pmatrix} 2 \\ 10/\sqrt 2 \end{pmatrix}=\begin{pmatrix} 2 \\ 5\\ 5 \end{pmatrix}


2.

x240    x2x^2-4\ge 0 \implies |x|\ge 2

then image of S:

f(x)2(2)1=5f(x)\le 2(-2)-1=-5 and

f(x)221=3f(x)\ge 2\cdot 2-1=3


3.

for ker(T):

U+V=0U+V=0

U+2U=0U+2U=0

so,

ker(T)=(0,0)ker(T)=(0,0)


4.

by the rank-nullity theorem:

6=dim(Ker(T))+dim(Im(T))6=dim(Ker(T))+dim(Im(T))

So, dim range T= 6-dim(Ker(T))=6-2=4


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